REACTION OF CH4 AND CH3F WITH SI(111)

Ab initio configuration interaction theory is used to study the reacti ons of CH4 and CH3F on a cluster model of the Si(111) surface. The lat tice is modeled as a three layer cluster of 13 Si atoms plus 27 H atom s. The calculated heat of reaction for CH4 + Si(111) --> CH3 + H-Si(11 1) is endothermic by 13 kcal mol(-1) with a transition state energy ba rrier of 24 kcal mol(-1). The reaction CH3F + Si(111) --> CH2F + H-Si( 111) is 12 kcal mol(-1) endothermic with an energy barrier of 20 kcal mol(-1). The reaction CH3F + Si(111) --> CH3 + F-Si(111) is 24 kcal mo l(-1) exothermic, with an energy barrier of 19 kcal mol(-1). The F, H, CH3, and CH2F species are found to bond strongly to the Si(111) surfa ce with adsorption energies of 132, 86, 78, and 76 kcal mol(-1), respe ctively. Equilibrium geometries: vibrational frequencies and transitio n state geometries for the transfer of H and F in reactions of CH4 and CH3F with Si(111) are reported.